Security breaches that expose sensitive customer data are becoming uncomfortably common these days. The newest firm to fess up to falling prey to a hacker attack is AT&T, which has begun sending letters out to customers letting them know about a breach that occurred between April 9 and April 21 of this year.

According to a report in Dallas News, the attack resulted in an undisclosed number of wireless customers having their personal data exposed, including Social Security numbers and dates of birth -- handy information if you're trying to steal and/or sell someone's identity.

Luckily, it doesn't appear that identity theft was on the radar of the cybercriminals involved. Instead, the hackers simply wanted to spoof existing AT&T customers so they could unlock a bunch of old handsets.

"AT&T believes the employees accessed your account as part of an effort to request codes from AT&T that are used to ‘unlock’ AT&T mobile phones in the secondary mobile market," AT&T stated in its letters to consumers.

If that's the case, this breach is far less concerning (for consumers) than it could have been, though there's still that uneasy feeling of having your personal data exposed. It looks like this is a backlash against AT&T's tightened policies regarding the ability to unlock handsets, which can only be done at the beginning or end of two-year contracts.